364 



J'HYSIOLOGY OF THE INVERTEBRATA. 



with the base of a crystalline cone. Each cone passes 

 inwardly into a nerve-rod, and then thickens into a striated 

 spindle-shaped body. The inner extremities of the striated 

 spindles become narrow again, and then pass into the optic 

 nerve (Mg. 70). 



k 



Fig. 70. — The Eye of the Decapoda. 



A = Eye of Astacus. B = Eye of Homarus. a = cornea, b = crystalline 

 cones. c = nerve rods. d '= striated bodies. e = optic nerve. 

 /= lenticular bodies, g- = fenestrated membrane. A = layer not present 

 in Astacus. k = pigment cells between cones. C = cornea of Decaf oda. 

 D = cornea of Insecia. 



There are certain species of crayfishes which are blind; 

 among these may be mentioned Gambarus setosus (Faxon), 

 which lives in the caves of south-western Missouri, and 

 Camhartis pellucidics, the well-known species from the Mam- 

 moth Cave. Mr. G. H. Parker* has recently examined the 

 question of degeneration of these organs. He states that 



* Bulletin of the Museum of Comparative Anatomy at Harvard College, 

 vol. 20 (1890). 



